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Isolation and Regulation Transformer Operating Principles and Transients
Response©
By Karl B. Clark, E.E., E.I.T.
Power Quality Consultant
I. O p erati n g Pri n ci p a l s of Wi d el y U s ed T r an sf o r me r s
Power distribution transformers, as well as the power
supply transformers of individual pieces of equipment and
appliances, provide isolation. Isolation means that the
transformer primary voltage and current, the input to the
transformer, is physically separated from the transformer
secondary current and voltage, the output of the transformer.
Energy is transferred from the transformer primary to the
transformer secondary via the changing magnetic field which
links the transformer primary and secondary windings.
When the transformer primary current changes, it causes the
Typical isolation transformers
magnetic flux or magnetic lines of force to change. This
changing magnetic flux or magnetic field causes a voltage to
be induced across the secondary windings. When an electrical load is connected across the secondary
windings of the transformer, the voltage induced into the secondary windings will cause a current to flow
through the connected load. There is no direct physical connection between the primary and secondary.
The common magnetic field connects the primary and secondary. The set of current, magnetic field, and
induced voltage relationships is often referred to as transformer action. Transformer action denotes that
transformers operate by mutual inductance. The relationship between the voltages, currents, and number
of turns of wire on the transformer primary and secondary for an ideal, lossless, or 100 per cent efficient
transformer are shown in Figure 1 and Equation 1 below.
Secondary
Primary
Ferromagnetic Core
Magnetic Flux Linking
Primary and Secondary Windings
Figure 1. Simplified Transformer Diagram
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Equation 1. Transformer Relationships.
n1/n2 = E1/E2 = I2/I1
n2 = the number of turns of wire on the primary
n2 = the number of turns of wire on the secondary
E1 = the voltage across primary
E2 = the voltage across secondary
I1 = the current through the primary
I2 = the current through the secondary
When n1 is greater than n2, we have a voltage step down transformer. For example, if n1 = 400,
n2 = 200 and E1 = 480 volts; from Equation 1 and solving for E2, we have:
E2 = (E1 X n2)/(n1) = (480 V X 200)/(400) = (96,000 V)/(400) = 240 volts
Note that the voltage is stepped down from 480 volts to 240 volts. From Equation 1, observe
that when the voltage is stepped down the current is stepped up and vice versa. The ratio of primary
power to secondary power is equal to one for a lossless transformer. Transformers can not create
energy. The number of turns on the transformer primary and secondary can be varied to obtain desired
voltage or current step up or step down. Transformers are also available with multiple taps or
connections at different points along the primary and/or secondary windings. These transformers can
be used to correct constant under or over voltage supply conditions by utility engineers or facility
managers. For example, if a facility is receiving a constant 204 volts and it requires a constant 240
volts, a suitably adjusted multiple tapped transformer will allow the low voltage condition to be
corrected by selecting the correct tap.
Electronic tap-switching voltage regulators utilize the multiple tapped primary winding principle
to maintain a relatively constant voltage to a load under changing input voltage conditions. Typically,
voltage sensing and control circuitry monitor the transformer secondary voltage and electronically
switches taps on the primary winding in an attempt to maintain a constant output voltage. Electronic
tap-switching voltage regulators are suitable for reducing supply voltage problems which are of
relatively long duration such as utility brownouts caused by high peak demand. Because of their slow
response time, which may be several electrical cycles, they are unable to respond to shorter
fluctuations and to transients. Tap-switching voltage regulators create transients when they change
taps at points other than the zero crossing of the output current waveform. Any abrupt current change
in an electrical circuit produces an abrupt change in the magnetic flux linking the circuit. The
changing magnetic flux produces an induced voltage in the circuit. The self-induced voltage is of a
polarity which opposes the change in current which produced it. This is indicated by the negative
sign in Equation 2 below.
Equation 2. Induced Voltage Formula.
E = -L di/dt, where
E = self-induced or back emf in volts
L = self-inductance in henrys
di/dt = the time rate of change of the current in the circuit.
Suppose the primary current in an isolation transformer powering a sensitive load is suddenly
interrupted. Example 1 below, demonstrates the calculation of an isolation transformer collapsing field
transient. These transients are generated by the collapsing magnetic field of a transformer when the
primary current is interrupted (i.e., the transformer is switched off, a power failure, a breaker trips, a fuse
clears, etc.). They are generated when the primary current is interrupted at any point on current sine wave
except at a current zero crossing point. Thus, the odds greatly favor transient creation as there are only two
zero crossings for each complete three hundred and sixty degree sinusoidal cycle of the current waveform.
Example 1. What is the magnitude of the collapsing field transient created by the interruption of a
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twenty ampere current which collapses in 0.0021 seconds (approximately, one-eighth of a cycle)? The
mutual inductance, which relates the induced voltage in the secondary to the rate of change of current in
the primary, is 0.1 henries. From Equation 2 and the given information:
E = -L di/dt, where
L = 0.1 henries
di = 20 amperes
dt = 0.0021 seconds (approximately one-eighth of a cycle)
Substituting the given values into Equation 2, we have:
E = - (0.1 henries) (20 amperes / 0.0021 seconds)
E = - 952 volts, which is lethal to sensitive electronics.
The above example demonstrates that even small mutual inductances, small transient currents, and
slow pulse decay times are capable of generating destructive transients.
There are a variety of other isolation transformer designs available which are sold to reduce noise
problems. These include saturated transformers, ferroresonant transformers, and ultra-isolation
transformers.
Saturated transformers are designed to saturate their magnetic transformer core. When the
transformer magnetic core is saturated, further increases in the magnetic flux linking the primary and
secondary windings are limited as additional increases in primary current occur. Thus, normal mode
transients can be reduced. At full transformer saturation, primary transients which increase the peak
primary current will not be able to produce proportional increases in the magnetic flux linking the
secondary. Without a proportional increase in magnetic flux, there will not be a proportional increase in
the voltage of the secondary side of the transformer due to the transients in the primary circuit. The AC
excitation of the transformer by the alternating primary current causes the flux density in the
transformer's magnetic core to increase as the current increases and decrease as the current decreases.
When the current sine wave crosses the zero point and increases in the negative direction, the flux density
will also change polarity or direction. The transformer's flux density depends upon the peak value of the
primary current at any instant for its magnitude. The polarity of the flux density is determined by the
polarity of the primary current. At the positive peak of the current sine wave, the flux density will
achieve its maximum positive value and the transformer will be saturated if it is properly loaded. When
the negative peak of the primary current sine wave occurs, the flux density will attain its maximum
negative value. Transient currents, which appear at the transformer primary in between the positive and
negative current peaks, will be able to produce changes in the flux density. This will cause transient
voltages to appear across the transformer secondary windings by normal transformer action. Since it is
unlikely that a significant percentage of transients will appear at the positive current peak of 90 degrees
or the current negative peak of 270 degrees, it is reasonable to expect that a significant portion of the
transients will appear at the secondary. Even when operating exactly as designed, saturated transformers
provide no common mode noise or common mode transient protection.
Transformers can pass transients and noise directly from their primary to secondary via primary to
secondary parasitic capacitive coupling. This parasitic capacitive coupling is an undesired consequence of
transformer construction. Two conductors which are separated by an insulator or dielectric form a
capacitor or condenser. The insulator or dielectric can be the insulating varnish on the wires of the
primary and secondary transformer windings, air, mica, or a variety of plastic films and other materials
selected to achieve special capacitor or condenser characteristics. Equation 3 below, demonstrates that
the current flow through a capacitor is dependent upon the capacitance (measured in farads) and the rate
of change of voltage with respect to time.
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Equation 3. Capacitor Current.
I = C dv/dt, where
I = the current through the capacitor
C = capacitance in farads
dv/dt = the rate change of voltage with respect to time
As the capacitance increases, the current also increases for a constant dv/dt. And, as dv/dt
increases, for a constant capacitance, the current will also increase. For a DC voltage, (a constant or
unchanging voltage) dv/dt is zero and the current is also zero. Thus, capacitors block direct currents. As
dv/dt increases or, as sinusoidal frequency increases, the current also increases for a constant
capacitance. Thus, a capacitor blocks DC and passes AC. Because a capacitor is constructed with
conductors separated by an insulator, no physical current actually flows through the capacitor's insulator
or dielectric. As the voltage across the capacitor increases, electrical charges of opposite polarity build
up on the opposing conductors. This build up of opposite charges creates an electric field between the
two conductors. The electric field strength, which is usually measured in volts per meter, is proportional
to the charge stored in the capacitor, and is also proportional to the voltage across the terminals of the
capacitor. Thus, capacitors are capable of storing electrical energy.
Returning to the discussion of saturated transformers, they will pass transients and noise from the
primary to secondary via the parasitic capacitance between the primary turns and the secondary turns, as
well as by normal transformer action when the transformer is not fully saturated. Additionally, saturated
transformers must be carefully selected and operated at their rated load to provide the design benefits.
Again, no protection against common mode noise is provided and by transformer action a rapidly
collapsing magnetic field due to a power failure, or fuse clearing can generate lethal transients at the
secondary which will be applied to sensitive connected loads.
Ultra-isolation transformers are capable of achieving substantial reductions in primary to
secondary parasitic capacitance by wrapping the primary winding and secondary winding in conductive
nonferrous metal foil, such as copper or aluminum and bonding it to ground. This will reduce the
parasitic capacitance but will not effect normal transformer action. This is true because copper and
aluminum are nonmagnetic materials. Now the primary winding parasitic capacitance is formed
between the primary winding and the grounded primary metal foil forming a capacitor with one lead
grounded. This tends to short circuit high frequency noise to ground. The same arrangement can be
employed for the secondary winding. When properly operated and loaded, ultra-isolation transformers
will saturate and provide some common and normal mode noise protection and some transient
protection. Ultra-isolation transformers are frequency sensitive and load sensitive. If they are not
operated in saturation, they provide little benefit. Additionally, ultra isolation transformers tend to be
large, noisy, and they generate heat. The generated heat is a double negative as the heat generation
increases utility power bills and the air conditioning systems must work harder to remove the heat which
increases utility power bills a second time. Short-term power losses, drop outs, fuse clearings and
similarly rapid current changes in an ultra-isolation transformer will generate transients according to
Equation 2 as previously mentioned.
Ferroresonant transformers are also known as constant-voltage transformers or ferroresonant
voltage regulators. Typically they utilize a combination of magnetic components (such as a specially
designed transformer) and a capacitor. The combination is tuned to the input power frequency. They are
used to regulate the output voltage as the input voltage changes. Open-loop (no feedback system)
ferroresonant voltage regulators do not sample their own output voltage to automatically correct it for
slow variations in input voltage. The regulating ability of open-loop ferroresonant regulators is dependent
upon the frequency stability of the power source, the magnetic characteristics of the design, and the load
impedance. Highly capacitive loads may detune the ferroresonant regulators causing loss of voltage
stability and regulation. Closed-loop (with a feedback system) ferroresonant voltage regulators utilize
feedback to adjust their output voltage. By feedback, we mean that the actual output voltage is compared
to a reference voltage in the control circuitry of the closed-loop regulator. Based upon the comparison,
the output voltage is increased or decreased as required.
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When properly designed, sized, and operated with the correct load some reduction in normal
mode transients should occur. Typically, ferroresonant transformers are not effective against common
mode transients. They are also capable of generating harmonics because their output is not a pure sine
wave. The output must be properly filtered to remove undesired harmonics. Ferroresonant transformers
will also generate collapsing field transients.
Another family of voltage regulating transformers is the boost and buck transformers. These
transformers are constructed in such a manner that the primary windings are capable of increasing
(boosting) or decreasing (bucking) the magnetic flux linking the transformer secondary. This increases or
decreases the secondary voltage as the input voltage or load on the secondary changes, providing voltage
regulation. A variety of boost and buck regulators are available including those which employ solid-state
control circuitry, such as thyristors, to improve voltage regulation. Typically, transients will pass from the
primary to the secondary of a boost and buck transformer because of the normal transformer action and
parasitic capacitive coupling between the primary and secondary windings.
II. Isolation Transformer Transient Response Tests.
A TCM brand medical grade toroidal core (donut
shaped magnetic core) isolation transformer with a
static shield and a standard T-U brand laminated
core (the core is shaped like Figure 1 and built with a
series of steel plates stacked on top of each other or
laminated together) laboratory type isolation
transformer were each subjected to standard
ANSI/IEEE C62.41-1991 transient test waveforms.
Normal mode (line-to-neutral) frequency response
measurements to MIL-STD-220A were also made.
Figures 2, 3, and 4 below provide the normal mode let-through voltages at the secondary of the
TCM brand isolation transformer for applied A3 and B3 ring waves and the B2 combination wave (8 X 20
µsec impulse).
As shown in Figure 2. below, the TCM (medical grade isolation transformer) when subjected to an
ANSI/IEEE Std. C62.41-1991, Category A3 Ring Wave (oscillatory frequency of 100 kHZ, 6,000 V, 200
A) produced a let-through voltage of 2,230 volts (2.23 kV). This high magnitude of let-through voltage
is capable of disrupting, damaging and possibly destroying costly and sensitive electronic systems.
Additionally, the cumulative effect of such transients degrades the electrical distribution system
insulation and the loads which are connected to the distribution system.
The A3 Ring Wave is typical of an outlet level transient. That is, it is not a severe standard
ANSI/IEEE transient. The voltage attenuation ratio and attenuation in decibels (db) for this A3 Ring
Wave is:
Equation 4. Voltage Attenuation Ratio.
Voltage Attenuation Ratio = Vout/Vin = 2,230 V / 6,000 V = 0.3717
Equation 5. Attenuation in db.
Attenuation (db) = - 20 log10 (Vout/Vin), where the “-” sign is used to convert the attenuation in db to
a positive number.
Attenuation (db) = - 20 log10 (Vout/Vin) = -20 log10 (0.3717) = (-20)(-0.4298) = 8.60 db. Note that
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isolation transformer manufacturers typically specify attenuation in the 40 db range and up. This
specification is a frequency domain characteristic. Due to the measurement techniques employed, the
advertised attenuation will not be realized (in either the time domain or frequency domain) in actual
practice.
Figure 2. TCM transformer output of 2,230 volts (2.23 kV) for the applied A3 Ring Wave transient (100
kHZ, 6,000 V, 200 A).
Figure 3. below presents the oscilloscope trace for the output transient of 2.30 kV for the Category B3
Ring Wave input transient (100 kHz, 6kV, 500 A).
As shown in Figure 4 below, the TCM isolation transformer does not provide transient protection by
reducing transients to a safe level. The TCM isolation transformer functioned as a transient amplifier
increasing the amplitude of the B2 impulse from 4,000 volts to 5,750 volts. The TCM isolation
transformer is not a transient cure.
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Figure 4. TCM transformer output of 5,750 volts (5.75 kV) for the applied B2 Impulse (4,000 V, 2,000
A, 8 X 20 µsec). This is an example of transient voltage amplification (i.e., 4,000 V in and 5,750 V out).
The isolation transformer to the left is the T-U
transformer.
Figures 5, 6, and 7, below, provide the normal
mode (line-to-neutral) let-through voltages at the
secondary of the T-U brand isolation transformer
for applied A3 and B3 ring waves and the B2
combination wave (8 X 20 µsec. impulse).
Figure 5. T-U Transformer Output for an applied A3 Ring Wave (100 kHz, 6,000 V, 200 A)
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Figure 6. T-U Transformer Output for an applied B3 Ring Wave (100 kHz, 6,000 V, 500 A)
Figure 7. T-U Transformer Output for an applied B2 Impulse (4,000 V, 2,000 A, 8X20 µs)
As shown in Figures 5, 6 and 7, the T-U isolation transformer does not provide transient protection
by reducing transients to a safe level. The T-U isolation transformer functioned as a transient amplifier
increasing the amplitude of the A3 ring wave from 6,000 volts to 8,780 volts, the B3 ring wave from
6,000 volts to 8,890 volts and the B2 impulse from 4,000 volts to 6,560 volts. Clearly, the T-U isolation
transformer is not a transient cure.
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III. Isolation Transformer Transient Response Summary and Comparison with the
Surge Suppression Incorporated® CKL1S1
Table 1 below provides the comparative transient suppression effectiveness or let-through voltages, for
the TCM and T-U isolation transformers and the Surge Suppression Incorporated® CKL1S1
Products / Let-through Voltages:
ANSI/IEEE C62.41-1991 Standard
Transient Test Waveforms. All tests
are in the line-to-neutral mode.
Transients are applied at the 90O point
of the power frequency sine wave
unless otherwise indicated.
TCM
Isolation
Transformer
T-U
Isolation
Transformer
Surge Suppression
Incorporated®
CKL1S1
(3 wire + ground,
120/240 V, split
phase, 40 kA per
mode, SWT)
Average Per
Cent
Improvement
With CKL1S1
Al Ring wave (100kHz, 2,000V,
67A)
1,390 V
2,880 V *
A2 Ring wave (100kHz, 4,000V,
133A)
1,890 V
6,080 V *
N/A
N/A
A3 Ring wave (100kHz, 6,000V,
200A)
2,230 V
8,780 V *
N/A
N/A
B1 Ring wave (100kHz, 2,000V,
167A)
1,420 V
2,980 V*
N/A
N/A
B2 Ring wave (100kHz, 4,000V,
333A)
1,940 V
6,210 V *
N/A
N/A
B3 Ring wave (100kHz, 6,000V,
500A)
2,300 V
8,980 V *
N/A
N/A
B1 Impulse (2,000V, 1,000A,
8X20µs)
3,300 V *
3,190 V *
N/A
N/A
B2 Impulse (4,000V, 2,000A,
8X20µs)
5,750 V *
6,560 V *
N/A
N/A
B3/C1 Impulse (6,000V, 3,000A,
Failed,
insulation
breakdown
9,800 V *
8X20µs)
28 V @ 270O
390 V @ 90O
7,525 %
2,413 %
Note: Data for
T-U only,
the TCM failed
* Note: The let-through voltage outputs exceeded the transient voltages inputs for these cases.
While isolation transformers are widely sold as transient cures, the test data in Table 1 above
shows that these isolation transformers are not transient cures. And, they can actually make the transient
situation worse. For the B2 and B3/C1 impulses, the isolation transformers functioned as transient
amplifiers increasing the amplitude of the applied transient. The CKL1S1 reduced the transients to
harmless levels, while the isolation transformers passed lethal transients. The improvement obtained in
transient suppression with the CKL1S1 as shown in the average per cent improvement column above
demonstrates the superior performance of the CKL1S1 and the danger of attempting to solve a transient
problem with an isolation transformer.
VI. Isolation Transformer Frequency Response Tests
The normal mode (line-to-neutral) secondary or output frequency response data (attenuation in
decibels at the indicated frequencies) for the TCM and T-U isolation transformers and the Surge
Suppression Incorporated® CKL1S1 are provided in Table 2. The measurements are to MIL-STD-220A,
the 50 ohm insertion loss measurement technique. This is the standard for EMI/RFI filter attenuation
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measurements. MIL-STD-220A uses a 50 ohm resistive signal generator internal impedance and a 50
ohm resistive load impedance. While this standard provides a yardstick to compare EMI/RFI filters, the
data is essentially meaningless for ac power distribution systems. Typically, the ac power source has a
complex (resistive, inductive and capacitive) internal impedance on the order of milliohms and the
complex load impedance is on the order of ohms. Thus, under MIL-STD-220A, an attenuation of 40 dB
becomes an attenuation of 0 dB at normal ac power distribution impedance levels. Suppose that a home
draws 100 amperes and that the internal impedance of the source (serving transformer) is 50 ohms
resistive. This means that the source would dissipate: P(in watts) = i2 r = (100 A)2(50 ohms) = 500,000
watts.
This would melt down the transformer and make electrical energy distribution, as we know it,
impossible. The point is that “attenuation figures” for filters and transformers should be viewed with
skepticism. The exact measurement techniques and their relevance to ac power distribution systems must
be clearly understood.
Surge Suppression Incorporated® CKL1S1 attenuation data is provided in Table 2 below per MILSTD-220A (the 50 ohm insertion loss method). The CKL1S1 is 120/204 V, split phase, 3-wire plus
ground, 40,000 peak surge amperes per mode (IEEE 8X20 µs impulse), panel mounted transient voltage
surge suppressor (TVSS). The CKL1S1 is also a sine wave tracking TVSS and is a logical alternative to
120/240 V isolation transformers which supply a load or secondary current of up to 800 amperes where
transient suppression and noise attenuation is desired.
Products/Attenuation in Decibels
Frequency
10 kHz
Surge Suppression
Incorporated®
TCM Isolation T-U Isolation
CKL1S1
Transformer
Transformer
5.7 dB
1.70 dB
19.0 dB
100 kHz
20.0 dB
11.50 dB
38.0 dB
1 MHz
26.0 dB
40.00 dB
23.8 dB
10 MHz
16.6 dB
15.00 dB
11.7 dB
100 MHz
1.4 dB
9.10 dB
6.4 dB
Table 2. Frequency Responses For The TCM, T-U And CKL1S1
Average %
Improvement With
CKL1S1
+414 %
+141 %
-28.0 %
-26 %
+22 %
Isolation transformers are widely sold as noise cures. As shown above, the CKL1S1 performs well
when compared to isolation transformers. At lower frequencies, noise tends to pass from the primary of a
transformer to secondary by normal transformer action. At higher frequencies, primary to secondary
parasitic capacitive coupling tends to be dominating noise transfer mechanism.
Isolation transformers are correctly applied to eliminate detrimental ground loops (common mode
problems) and to establish a new neutral-to-ground bond where permitted by the National Electrical Code
(NFPA 70). A ground loop is formed when two or more points (or pieces of electrical equipment) in an
electrical system that are normally at ground potential are connected by electrical paths so that they are
not at the same ground potential. This occurs when “dedicated ground rods” and bonding to building
steel creates different physical and electrical ground points in a system. Additionally, the connection of
systems to data, control, telco lines and CATV can create ground loops due to their different physical and
electrical grounding points. Ground loop elimination techniques are often used to float laboratory
equipment especially when high voltages are in use. In order to guarantee all ground loops are
broken each piece of equipment requires its own isolation transformer. A simpler and less costly
cure when floating equipment is not required is to establish proper surge reference grounds (ground
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windows and local ground windows). An example of a local ground window is the application of a
corded sine wave tracking TVSS with RI-11 telco protection. The AC and telco lines are referenced to
the same ground potential allowing the computer, modem, monitor, printer, and telco lines to be properly
referenced and protected against the same local ground reference.
V. Transformer and Regulator Transient Creation and Transmission
Mechanisms with Recommended Protection Methods
There are a variety of transformers and transformer based devices in use in electrical distribution
systems. Because the basic operating principles of the transformer apply to these devices, their behavior
when subjected to transients will be similar to the transformers described above. Table 3 lists the more
common transformers and transformer based devices, their transient behavior, and provides a
recommended transient protection strategy.
Regulator or
Transformer
Type
Utility power
distribution
transformers
Electronic tapswitching
regulators. Note:
transients will be
created unless
switching occurs at
current zero crossing.
Saturated isolation
transformers
Creates
Collapsing
Field
Transients?
Passes
Transients by
Transformer
Action?
Passes
Transients via
Parasitic
Capacitance?
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
Recommended Protection for
Transformer of Sensitive Connected
Loads
Transformer protection as supplied
by utility. User service entrance and
interior distribution system should
be protected.
Provide protection at secondary or
output. And, if electronic tapchanging control circuitry is failing,
also protect the primary or input
with true all mode protection.
Provide protection at the
secondary or output.
Ultra-isolation
transformers
Yes
Yes
Yes
Ferroresonant
transformers or
regulators
Yes
Yes
Yes
Provide protection at the secondary
or output.
Provide protection at the secondary
or output. Use threshold clamping
surge suppressors. Do not use sine
wave tracking units.
Yes
Yes
Yes
Provide protection at the secondary
or output.
Yes
Yes
Yes
Boost and buck
transformers and
regulators
Facility distribution
transformers (i.e.
step-down, step-up,
Wye to Delta, Delta
to Wye, Delta to
Delta, Wye to Wye
Provide protection at the secondary
or output. When a transformer feeds
a panel or panels apply protection
at the panels to maximize protection
from transformer pass through
transients, transformer generated
transients, and transients fed back to
the panel (s) from connected loads.
Table 3. Transformer and Regulator Transient Creation and Transmission Mechanisms with
Recommended Protection Methods
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Never rule out transient or noise problems when a transformer or transformer based device is in
use. Transients can be amplified by transformer based devices. Transients and noise can pass
through transformers and transformer based devices. Additionally, transformer based devices can
create collapsing field transients. Establish proper surge reference grounds (ground windows and
local ground windows) per ANSI/IEEE Std. 1100-1999, the Emerald Book, and reference your
Surge Suppression Incorporated® surge and transient suppressors (AC, telco, data, and coax) to
these surge reference grounds.
Usage of this work is conveyed to Surge Suppression Incorporated (and affiliated companies) and
their staff and clients for their exclusive use. Any and all other uses including; but, not limited to
the reproduction and/or distribution in any and all forms are forbidden without the expressed written
permission of the author.
All data has been derived from sources that are believed to be reliable and source documents are on
file. The author assumes no liability or responsibility for specification changes, typographical
errors, or omissions.
© 2004 Karl B. Clark, All Rights Reserved.
Rev. 07
12/14/2004